Assign¶

Name a value; amend a named value

Simple assign¶

x:y

Where x is a name and y is a value, the value of y is associated with the name x.

q)a:42        / assign
q)a
42
q)a:3.14159   / amend

The Equal operator = tests equality. It has nothing to do with naming or amending values.

There is no need to declare the type of a variable.

A variable acquires the type of the value assigned to it. (Known as dynamic typing.)

q)type a:til 5    / integer vector
7h
q)type a:3.14159  / float atom
-9h

Indexed assign¶

x[i]:y

Where

x is the name of a list, dictionary or table
i is a value that indexes x (including a ;-separated multi-level index)
y is a scalar, or a list of the same shape as i

the value of y is assigned to x at indexes i.

Indexed assignment to a simple list cannot change the type of x.

If x is a simple list (has type between 1 and 20) then (=). abs type each(x;y) must be true. However, a general list that ends up having all items of the same atomic type after assignment will collapse into a simple list.

Where x is a dictionary, assignment has upsert semantics.

q)s:1 2 3
q)s[1]:4
q)s
1 4 3
q)s[2]:5f
'type
  [0]  s[2]:5f
           ^

q)s:(1;2f;3)
q)s
1
2f
3
q)s[1]:4
q)s
1 4 3
q)s[1]:5f
'type
  [0]  s[1]:5f
           ^

q)m:(1 2;3 4)
q)m[1;1]:5
q)m
1 2
3 5

q)d:`tom`dick`harry!1 2 3
q)d[`dick`jane]:100 200
q)d
tom  | 1
dick | 100
harry| 3
jane | 200

Assign through operator¶

x op:y     op:[x;y]
x[i]op:y

Where

op is a binary operator with infix syntax
x or x[i] is an assignable in the left domain of op (the rules of indexed assign apply for x[i])
y is a value in the right domain of op that conforms to x (or i)

the value of x (or x[i]) becomes x op y (or x[i]op y).

q)s:("the";"quick";"brown";"fox")
q)s[1 2],:("er";"ish")
q)s
"the"
"quicker"
"brownish"
"fox"

Extend Assign-through-operator to derived functions, keywords and lambdas.

q)s:("the";"quick";"brown";"fox")
q)@[s;1 2;,;("er";"ish")]
"the"
"quicker"
"brownish"
"fox"

Amend At is more general, and extends assignment-through-operator to derived functions, keywords and lambdas.

If x is undefined, the identity element for op is used as a default.

q)bar
'bar
  [0]  bar
       ^
q)bar+:1
q)bar
1

Some operators have significant differences between their base and assignment form, for example ,.

Pattern match¶

See Pattern matching

Syntax¶

An expression with an assignment on the left returns no value to the console.

q)a:til 5
q)

The value of an assignment is the value assigned.

q)3+a:til 5
3 4 5 6 7
q)1+a[2]+:5
8
q)a
0 1 7 3 4

Pick second argument¶

If : is used as a value (e.g. assigned to a variable, passed as a function parameter, or modified by an iterator), invoking it causes it to discard its first argument and return its second argument, without performing an assignment.

q)a:3
q)f:(:)
q)f[a;4]
4
q)a
3

A common use of this is with functional amend, where it indicates replacing the element at the specified index with a new value, as opposed to performing an operation between the existing and new values:

q)@[1 2 3 4 5; 2; :; 6]
1 2 6 4 5

Combining it with over has an effect equivalent to last:

q):/[1 2 3 4 5]
5

Amend, Amend At
Q for Mortals §4.6.2 Simple q Amend